Damping material of an improved formulation for use on a magnetic head

ABSTRACT

A damping material (19) is provided on one of the two side faces (7, 8), of a magnetic head having two plate shaped ferrite core limbs (3, 4). The material consists of a synthetic resin filled with less than 30% by volume of filler having a density of more than 10×10 3  kg/m 3 . The filler is at least 74% by weight of the damping material and the density of the damping material is larger than 4×10 3  kg/m 3  and its modulus of elasticity ranges between 10 7  and 10 9  Pa.

BACKGROUND OF THE INVENTION

The invention relates to a magnetic head having two combinedplate-shaped ferrite core limbs with a tape contact face in which a gapbounded by the two core limbs terminates. A damping material is at leastpartly provided on one of the two side faces constituted by the two corelimbs and extending transversely to the tape contact face. In such amagnetic head known from JP-A No. 55-84021 the damping material consistsof a porous and inorganic material. Such a damping material is used toreduce interference signals occurring in the output signal of a magnetichead, which interference signals are produced when a record carrier, forexample a magnetic tape is moved over the tape contact face of themagnetic head. In the Anglo-American literature such interferencesignals are also referred to as sliding noise or rubbing noise. Practicehas proved that the damping material described in JP-A No. 55-84021reduces such interference signals to an unsatisfactory extent.

SUMMARY OF THE INVENTION

The damping material is a synthetic resin filled with less than 30% byvolume of filler, which filler has a density of more than 10×10³ kg/m³and is at least 74% by weight of the damping material. The density ofthe damping material is larger than 4×10³ kg/m³ and its modules ofelasticity is between 10⁷ and 10⁹ Pa. In this manner the interferencesignals produced when a record carrier is moved over the tape contactface of the magnetic head can be substantially prevented. This is due tothe particular choice of the mixing ratios of the synthetic resin withthe filler having a large density so that a damping material is obtainedwhich itself has a large density but in which the particles of thefiller embedded in the synthetic resin still have an adequate mutualfreedom of movement in order to yield a satisfactory damping inco-operation with the synthetic resin. Due to the special choice of themodulus of elasticity of the damping material it is also achieved thatthe magnetic properties of the ferrite core limbs are not detrimentallyinfluenced as a consequence of tensions produced in the ferrite when thedamping material is cured, so that the magnetic head has invariableelectrical signal properties.

In this respect it is to be noted that a sealing compound for the corelimbs of a magnetic head accommodated in a housing is known from JP-ANo. 57-222.643, which compound inter alia also reduces the interferencesignals produced when a record carrier is moved over the tape contactface of a magnetic head and which consists of an epoxy resin providedwith a filler having a density of more than 5×10³ kg/m³, with 9.14×10³kg/m³ being given as the largest density and 6.95×10³ kg/m³ being givenas the smallest density in the examples. For the mixing ratio with thefiller, 50 to 200 pts.wt to 100 pts.wt of the epoxy resin are indicatedfor the filler, which corresponds to a part by weight of the filler ofbetween 33 and 66% by weight. If the resultant densities of the sealingcompound itself are calculated therefrom, values of between 1.6×10³ and2×10³ kg/m³ are obtained, hence relatively small densities. The relevantratios are indicated in the diagram shown in FIG. 1 in which the rangeof densities of the sealing compound resulting from the describedexamples is characterized by a cross-hatched area. This diagram alsoshows that only densities of at most approximately 3.2×10³ kg/m³ couldbe achieved for the sealing compound in the indicated mixing ratio forthe sealing compound, even if a filler having a density which is largerthan the density indicated in the examples were to be selected. Theachievable damping with such sealing compounds is also correspondinglysmall. Indications about the modulus of elasticity of such sealingcompounds cannot be derived from the JP-A No. 57-222.643. On the otherhand a damping material according to the invention is used with adensity of more than 4×10³ kg/m³ which is at least partly provided on atleast one of the two side faces of the magnetic head. In this manner theinterference signals produced when a record carrier is moved over thetape contact face of a magnetic head are suppressed very satisfactorily.

As can be derived from the relevant Handbooks, materials or alloys aresuitable as fillers having densities of more than 10×10³ kg/m³. It hasbeen proved to be very advantageous if the damping material comprisestungsten as a filler. In this manner the damping material can bemanufactured in a simple manner at low cost, whilst densities of up to6.6×10³ kg/m³ can be achieved for the damping material in the indicatedmixing ratios due to the relatively large density of 19.3×10³ kg/m³ oftungsten.

In this respect it is to be noted that a material consisting of a resinfilled with tungsten is known per se from U.S. Pat. No. 4,528,652 which,however, is used in connection with ultrasonic transducers for absorbingultrasonic soundwaves without reducing or damping the oscillationcapacity of the transducer, which is effected by a corresponding choiceof the acoustic impedance. An other possibility of using this material,more specifically in connection with magnetic heads and a damping of itsmechanical vibrations as in the case according to the invention, cannotbe derived from U.S. Pat. No. 4,528,652.

It has been proved advantageous if the density of the damping materialis chosen to be of the order of 5.2×10³ kg/cm³. With such a density ofthe damping material a uniform distribution of the particles of thefiller in the synthetic resin and a perfect cladding thereof by thesynthetic resin can be achieved, whilst the separate particles stillhave an adequate mutual freedom of movement to yield a satisfactorydamping in co-operation with the synthetic resin. In this connection theorder of magnitude is to be understood to have values of ±10%.

Furthermore it has been proved advantageous if the modulus of elasticityof the damping material is chosen to be of the order of 3×10⁸ Pa. With amodulus of elasticity thus chosen, which is conventionally effected byadding a softener to the synthetic resin, tensions in the core limbferrite produced during curing of the damping material are not too largeto detrimentally change the electrical signal properties of the magnetichead. In this respect the order of magnitude is to be understood to havevalues of ±50%.

It has also been proved advantageous if the damping material is providedin the form of locally bounded areas while leaving intermediate spacesin between the areas. A damping material thus distributed also reducesor prevents tensions in the core limb ferrite produced during curing ofthe damping material so that, as already stated, the electrical signalproperties of the magnetic head are not changed detrimentally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram illustrating the resultant relationships whenusing different mixing ratios and compositions of the damping material.

FIG. 2 is a side view of a conventional magnetic head unit having amagnetic head arranged on a carrier.

FIG. 3 shows the magnetic head unit according to FIG. 2 in a plan view.

FIG. 4 shows a magnetic head unit in which damping material is providedon all the freely accessible surface portions of the two side faces ofthe magnetic head.

FIG. 5 is a plan view taken on the line V--V in FIG. 4.

FIG. 6 is a bottom view taken on the line VI--VI in FIG. 4.

FIG. 7 is a plan view of a magnetic head unit in which damping materialis only provided at locally bounded areas on the freely accessiblesurface portions of the two side faces of the magnetic head.

FIG. 8 is a bottom view of the magnetic head unit shown in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 2 and 3 show a conventional magnetic head unit 1 which is,suitable to be provided on a rotatable drum and for scanning a magnetictape in obliquely extending tracks. The magnetic head 2 comprises twocombined plate-shaped core limbs 3 and 4 of ferrite constituting on anarrow side a tape contact face 5 for a magnetic tape in which a gap 6bounded by the two core limbs 3 and 4 terminates. Such a magnetic headhas two side faces 7 and 8 extending transversely to the tape contactface 5 and to the gap 6. With one part of the side face 7 the magnetichead 2 is provided by means of, for example adhesion on a side face 9 ofa carrier 10 so that its active part comprising the gap 6 projects fromthe free end 11 of the carrier 10. Contiguously to the area of the gap 6the magnetic head is conventionally provided with a coil winding 12which is passed through a coil window 13 provided in the two core limbs3 and 4 and whose two ends 14 and 15 are connected to leads 16 and 17formed as a printed circuit on a printed wiring board 18 which isarranged on the carrier 10.

If a magnetic tape is moved over the tape contact face 5 of the magnetichead 2 during operation of such a magnetic head unit, interferencesignals caused by mechanical vibrations of the magnetic head are knownto be produced in the output signal of the magnetic head. In order toreduce such interference signals, a damping material may be provided onat least one side face of the magnetic head, which material then dampsmechanical vibrations of the magnetic head.

It has been proved that the damping of such mechanical vibrations of themagnetic head 2 is particularly effective if a damping material having arelatively large density is used. To this end a synthetic resin filledwith less than 30% by volume of filler is used as a damping material, inwhich the filler has a density of more than 10×10³ kg/m³ and in whichthe part by weight of the filler is at least 74% by weight, whilst thedensity of the damping material is larger than 4×10³ kg/m³. An exampleof a synthetic resin which may be used for this purpose is an epoxyresin such as is available from the firm of Ciba-Geigy under thecommercial name "Araldit F" which has a density of approximately 1.2×10³kg/m³. For the mixing ratios stated, densities of the damping materialwhich are larger than 3.4×10³ kg/m³ can be achieved dependent on whichfiller having a density of more than 10×10³ kg/m³ is used such as, forexample lead or tantalum. Practice has proved that a satisfactorysuppression of the interference signals is achieved if the density ofthe damping material is more than 4×10³ kg/m³. The mixing ratios statedfor the synthetic resin with the filler having a density of more than10×10³ kg/m³ also result in the particles of the filler embedded in thesynthetic resin having an adequate mutual freedom of movement so thatthey yield a satisfactory damping in co-operation with the syntheticresin.

The diagram shown in FIG. 1 illustrates the relevant ratios. In thisdiagram the density of the damping material is plotted in 10³ kg/m³ onthe ordinate and the part by volume of the filler in the dampingmaterial is plotted in % by volume on the abscissa. The broken lines inthe diagram correspond to a given density of the filler used, whichdensity is indicated in 10³ kg/m³ and the dot-and-dash lines correspondto a given part by weight of the filler in the damping compound in % byweight. The density of the filler is top-limited by the materials havinga largest density of 22.5×10³ kg/m³ such as osmium or iridium. With theaforementioned mixing ratios a range of the selectable density of thedamping material indicated by means of a simple cross-hatched area inthe diagram is achieved.

It has been proved to be particularly advantageous if the dampingmaterial comprises tungsten as a filler which has a density of 19.3×10³kg/m³ so that densities of up to 6.6×10³ kg/m³ for the damping materialcan be achieved under the previously mentioned conditions, as can alsobe derived from the diagram shown in FIG. 1. Apart from the largedensities which can be achieved for the damping compound, such a dampingmaterial can be manufactured in a simple manner and at low cost. Withregard to a particularly satisfactory damping it has been proved to bevery advantageous if the part by weight of tungsten is chosen to beapproximately 82% by weight, by which a density for the damping materialof the order of 5.2×10³ kg/m³ is obtained. The particularly satisfactorydamping effect is based on the fact that with such a density of thedamping material a uniform distribution of the particles of the fillerin the synthetic resin and a perfect cladding thereof by the syntheticresin can be achieved, whilst the separate particles still have anadequate mutual freedom of movement in order to yield theafore-mentioned very satisfactory damping in co-operation with thesynthetic resin.

In the embodiment shown in FIGS. 4, 5 and 6 a damping material 19 asmentioned hereinbefore is provided on the freely accessible surfaceportions of the two side faces 7 and 8 of the magnetic head 2, with itsside face 8 being completely freely accessible in this case. In therelevant embodiment these surface portions are completely coated withthe damping material 19. It would of course alternatively be possible toprovide damping material only in part on these surface portions or toprovide the damping material on only one of the two side faces of themagnetic head. To this end the damping material 19 is provided in afluid state on the relevant surface portions and is subsequently cured,or in other words baked, in accordance with the requirements imposed onthe synthetic resin used. In this connection it has been proved to beessential that no or no excessive tensions in the ferrite of the corelimbs 3 and 4 are produced during curing of the damping material. Infact such tensions affect the magnetical properties of the ferrite andhence the electrical signal properties of the overall magnetic head. Inorder to obviate such unwanted influences it has been proved to beessential that the modulus of elasticity of the damping material rangesbetween 10⁷ and 10⁹ Pa, whilst it has been found to be particularlyadvantageous if the modulus of elasticity is chosen to be of the orderof 3×10⁸ Pa. The choice of such a modulus of elasticity of the dampingmaterial is conventionally made by adding a softener to the syntheticresin. In this manner the electrical signal properties of the magnetichead are not influenced detrimentally, whilst a satisfactory damping ofthe mechanical vibrations of the magnetic head is achieved so thatsubstantially no interference signals occur in the output signal of themagnetic head when a magnetic tape is moved over the tape contact face 5of the magnetic head 2.

To avoid tensions in the ferrite of the two core limbs 3 and 4 of themagnetic head 2, it has been found to be advantageous if the dampingmaterial 19 is provided only on locally bounded areas of the freelyaccessible surface portions of the side faces 7 and 8 of the magnetichead while leaving intermediate spaces between the areas, as is shown inthe embodiment according to FIGS. 7 and 8. In the relevant embodimentdamping material 19 is provided on two areas 20 and 21 of the freelyaccessible surface portion of the side face 7 and on four areas 22, 23,24 and 25 of the complete freely accessible side face 8, whilst theother surface portions remain free from damping material. The number ofareas on which damping material is provided and their location can ofcourse be varied. In the relevant embodiment the bounding of theseparate areas on which damping material is provided is circular, whichis achieved if the damping material is provided drop-wise. It is,however, alternatively possible to provide the damping material in astrip-shaped pattern or in a different pattern.

It is to be noted that embodiments of the entire magnetic head unitdifferent from those described are of course alternatively possible. Forexample, the carrier on which the magnetic head is provided may be adirectly rotatable drum. The measures according to the invention canalso be used for a magnetic head which itself is stationary or is movedradially with respect to a disc-shaped record carrier. As is evident theafore-mentioned embodiments may comprise a range of modificationswithout passing beyond the scope of the invention. In this connection itis also to be noted that the damping material can be provided on atleast one of the two side faces of the magnetic head before the magnetichead is mounted on a carrier. Likewise the damping material can beprovided on at least one of the two side faces of the magnetic headbefore its two core limbs are provided with a coil winding.

What is claimed is:
 1. A magnetic head comprising two combinedplate-shaped ferrite core limbs constituting on a side a tape contactface for a record carrier in which a gap bounded by the two core limbsterminates, a damping material being at least partly provided on atleast one of the two side faces constituted by said two core limbs andextending transversely to the tape contact face, characterized in thatthe damping material is a synthetic resin filled with less than 30% byvolume of filler, which filler has a density of more than 10×10³ kg/m³and is at least 74% by weight of the damping material and in that thedensity of the damping material is larger than 4×10³ kg/m³ and itsmodulus of elasticity ranges between 10⁷ and 10⁹ Pa.
 2. A magnetic headas in claim 1 wherein the damping material comprises tungsten as afiller.
 3. A magnetic head as in claim 1 wherein the density of thedamping material is chosen to be of the order of 5.2×10³ kg/m.
 4. Amagnetic head as in claim 1 wherein the modulus of elasticity of thedamping material is chosen to be of the order of 3×10⁸ Pa.
 5. A magnetichead as in claim 1 wherein the damping material is provided on only aselected portion of the side faces with other portions of the side facesbeing free from damping material.